The problem is that you create and render a combo box to an element that does not exist. From your code of rendering of the last column below:
- First, you generate a random id with Ext.id().
- Then you create a combobox and render this combobox to element with ID equals to generated id.
There is no other places to use this id. It means that there is no element with ID equals to generated id in your page at the time of creating and rendering combobox.